Certification authorities mandate that newly designed passenger aircraft shall have ignition reduction means applied to their fuel tanks. In the industry, this is commonly known as inerting. Inerting is typically implemented by filling the fuel tanks with oxygen depleted air (ODA) also known as nitrogen enriched air (NEA). Current passenger aircraft fuel tank ignition reduction regulations (FAR25.981) require that fuel tanks are kept inert with the ullage oxygen (O2) concentration at or below 12%.
Known inerting solutions include the use of on-board inerting gas generation systems (OBIGGS), which take an air supply from the aircraft engine bleed air system. The on-board inerting gas generation system commonly employ an air separation module (ASM), which comprise molecular sieves enabling the air to have a proportion of its oxygen filtered off and discarded, leaving ODA to inert the aircraft fuel tanks.
As passenger aircraft engines are typically mounted either on the aircraft wings, or in the aircraft tail region, ducting is required to provide a conduit for the bleed air from the engine, to the on board inerting gas generation system and subsequently to the fuel tanks, which may be located in the wings or in the centre wing box.
Bleed air taken from the engine is done so at temperatures in excess of 100 degrees Celsius (° C.). Therefore the ducting referred to above must be heat insulated, further increasing its weight and space requirements in the aircraft design architecture.
It is undesirable to supply ODA to the aircraft fuel tanks at temperatures in excess of 80° C., and coupled with the fact that the operating efficiency of an ASM is reduced at relatively high temperatures, the bleed air is commonly cooled. Known methods for cooling bleed air include routing the bleed air through a conditioning module prior to the ASM. This further ducting again adds weight and increased space requirements. The weight and power consumption of the conditioning module are also significant. Filtering of the bleed air is also necessary, to prevent contaminants or possibly harmful elements from entering the air separation module, adding weight and further complexity to the ducting arrangement.
The use of engine bleed air reduces engine efficiency, increasing fuel consumption, and limiting the thrust available from each engine.
U.S. Pat. No. 7,445,659 discloses a known method of inerting using a combination of ram air and engine bleed air, with an air separation module.
In U.S. Pat. No. 7,445,659 engine bleed air is used to provide a pressure differential such that the air separation module receives a sufficient flow of ram air. Effectively the ram air is ‘pushed towards’ the air separation module by the engine bleed air. As described above, when engine bleed air is used in U.S. Pat. No. 7,445,659, conditioning and filtering of the engine bleed air is required.
The present invention seeks to overcome these problems. It is the object of the present invention to provide an improved method of generating oxygen depleted air on an aircraft and an improved aircraft fuel tank inerting system.